A key concern for the operation of wireless communication devices is the acquisition of a coverage network with the highest probability of providing a good grade of service. The following discussion uses Code Division Multiple Access (CDMA) networks as an example, however, it should be understood that the discussion applies to other wireless communication networks. As a CDMA device approaches an edge of a CDMA coverage cell, the device begins operating at the limit of forward and/or reverse link budget of the CDMA network and a decision is required regarding an exit from the current cell. It is important to make an accurate determination of when to leave the current cell, as there are penalties associated with both premature and late exits from an existing wireless communication coverage network, hereafter referred to as an existing coverage network or a current coverage network/area.
Premature exit entails the unnecessary loss of the preferred coverage system and may entail the acquisition of an analog coverage network with attendant degradation of battery performance. Late exit may entail missing pages or dropping calls on origination due to reverse-link limitations.
It is known to allow wireless communication devices to remain on the existing coverage until complete loss of the paging channel. This approach typically results in late exits. It also is known to initiate the exit process when the device reaches a predetermined location. To implement this approach, a threshold boundary line or boundary zone at a predetermined distance from the base station is established by the base station (BS) or mobile switching center (MSC) for the coverage network providing service to the device. Typically, the BS also determines the location of the device. When the device moves past the threshold line or into the threshold zone, the device exits the existing coverage network.
FIG. 6 is a pictorial representation showing the path of a wireless device through an existing coverage network and an adjacent coverage network (prior art). The threshold boundary line/zone approach described above may result in premature exits from the existing coverage network. That is, a momentary excursion across the threshold line can cause the wireless communication device to exit the existing coverage area even when the device quickly returns to and stays in the original coverage network. For example, in FIG. 6, the device exits network 1 and acquires network 2 when moving from point A to B, but shortly thereafter, exits network 2 and re-acquires network 1 when moving from point B to C and then on to point D.
During each transition between coverage networks, data transmission can be impaired and extra battery power is consumed. In addition, each time the device switches from demodulating one Air-Interface to another as a result of the transition, the device also must reconfigure its resources to accommodate the new Air-Interface. The problem is exacerbated if the device ping-pongs, or oscillates, between coverage networks as a result of a series of rapid excursions across the threshold boundary line, for example, moving through points D to I in FIG. 6.
In a CDMA cell, the area of satisfactory operation (the aerial extent of the coverage area), and hence, the location of appropriate threshold boundary lines, is dependent upon the traffic within the cell. Due to inter-cell and intra-cell interferences, the base station noise floor may appear to rise as the traffic load within the cell increases. As a result, wireless devices in the cell must transmit additional power to overcome the increased interferences, effectively shrinking the cell. Shrinking the cell can move the area suitable for coverage network handoff closer to the center of the cell. However, threshold locations are typically fixed and cannot adapt to location changes to the area suitable for coverage network.
A need exists for a wireless communication device operating near a cell edge to accurately determine when to exit the existing coverage network so as to avoid premature exits, late exits, and oscillating between the existing and adjacent coverage networks.